Rain Barrel Calculator
Calculate rainwater harvest potential from your roof area. Get gallons per rainstorm and number of barrels needed.
๐ง๏ธ Rain Barrel Calculator
Results
Roof Area1,200.00 sq ft
Gallons per 1โณ Rain748 gal
Collectible per Storm599 gal
Barrels Needed11 barrels
๐ก How to Calculate Rainwater Collection
One inch of rain on 1 square foot of roof yields 0.623 gallons of water. A typical 30ร40 ft roof section produces 748 gallons per inch of rain. Collection efficiency is about 75โ85% due to evaporation, splash, and first-flush diversion.
Standard rain barrels hold 55 gallons. Position barrels under downspouts and connect overflow to a garden or drainage area. For larger systems, multiple barrels can be linked together. Collected rainwater is excellent for garden irrigation.
A 30ร40 ft roof section with 1 inch of rain at 80% efficiency: 1,200 sq ft ร 0.623 ร 0.80 = 598 gallons. You would need 11 standard 55-gallon rain barrels to capture it all.